There has been and still is a great amount of research involving surveillance at night and through camouflage in battlefield situations. This has led to the development of two distinct classes of optical devices known as image intensifiers and thermal viewers. The former make use of ambient skylight from the moon, stars or any low intensity sources of opportunity that happen to be near a target. The latter relies on blackbody radiation from the target itself or the deficiency thereof as compared with the ambient level of background radiation. Both devices rely rather heavily on the use of large apertures and associated precision optics to focus all of the light admitted by the aperture. The image intensifier also utilizes a broadband detector, i.e. one having a bandwidth greater than the human eye. The remaining intensification required for comfortable viewing is achieved by an electronic amplification of the signal supplied by the detector in these devices. The thermal viewer operates in a similar manner, but the detector is generally more sophisticated and operates entirely in the far-infrared region of the electromagnetic spectrum. Room temperature detectors are available, but high performance systems, i.e. large area high resolution types, still require special detector arrays with cooling (usually to the temperature of liquid nitrogen or less).
Even though these viewing devices are passive in nature, there has always been a concern that they might be spotted as targets by the enemy. Various designs have been devised to blend them into their background environments, but the optics must necessarily be exposed while such a device is in use. This leaves the device vulnerable to a support-measure employing a technique known as optical augmentation. This technique utilizes a hostile optical viewing system in the same manner as a corner reflector. A narrow beam of collimated light is directed into the optical system and becomes focussed on the first opaque surface encountered, usually the detector. A certain amount of the beam is also reflected from each transparent surface encountered. Light reflected from these surfaces tends to become refocussed into a beam directed back to the original beam source. A detector located at the source can then detect the presence of the viewer from the direction and intensity of the return beam relative to the beam transmitted.
The support-measure itself, however, has some serious drawbacks. Since the optical system should only be exposed while the viewing system is in operation, there is the definite possibility that the detector in the hostile viewer will pinpoint the collimated beam source, at least compromising its presence and inviting the destruction of the source and/or its operator. This is made even more hazardous if one assumes that both of the detectors involved are at least equally sensitive. Under these conditions the hostile detector will have at least twice the range of the support-measure detector.